TAS diagram
The TAS diagram (TAS = Total Alkali vs. Silica ) according to Mike le Bas is used to classify volcanic rocks (effluent rocks) based on the chemical composition of the total rock. The contents in percent by weight of Na 2 O + K 2 O in the Y direction are plotted against SiO 2 in the X direction.
This diagram is used when it is not possible to determine the modal mineral composition of the rock, as required for the classification in the route iron diagram . This is often the case with fine-grained or glassy volcanic rocks.
TAS diagram
The TAS diagram is divided into 15 fields. The names of the individual fields, i.e. H. the names of the rocks mostly have an equivalent in the route iron diagram. Fields with a red asterisk have two variants depending on the K / Na ratio. In detail, these correspondences are:
| Rock name | Abbreviation | comment |
|---|---|---|
| Foidite | F. | Name the most common foid if possible. Melilithite also falls into this area and can be distinguished by additional chemical criteria |
| Picrobasalt | Pc | |
| Basanite and tephrite | U1 | Differentiation through normative mineralogy ( CIPW norm or Barth-Niggli norm ) |
| Phonotephrite | U2 | |
| Tephrite phonolite | U3 | |
| Phonolite | Ph | |
| basalt | B. | Differentiation through normative mineralogy ( CIPW norm or Barth-Niggli norm ) |
| Trachy basalt | S1 | * The variants rich in sodium or potassium are Hawaiite and potassium trachy basalt |
| basaltic trachyandesite | S2 | * The variants rich in sodium or potassium are mugearite and shoshonite |
| Trachyandesite | S3 | * The variants rich in sodium or potassium are Benmoreite and Latit |
| Trachyte and trachydacite | T | Differentiation through normative mineralogy ( CIPW norm or Barth-Niggli norm ) |
| basaltic andesite | O1 | |
| Andesite | O2 | |
| Dazit | O3 | |
| Rhyolite | R. |
Here, sodium -rich means Na 2 O> K 2 O, high-potassium means K 2 O> Na 2 O
literature
- RW Le Maitre et al .: Igneous Rocks: A Classification and Glossary of Terms:: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. 252 pp., Cambridge University Press 2005. ISBN 978-0-521-61948-6 ( online )